The goal of this grant is to investigate the mechanisms controlling retinal cell differentiation in general and photoreceptor differentiation in particular. This competing renewal will investigate the role of transcription factors in the commitment of retinal progenitors to the photoreceptor fate, and in the divergent differentiation of rods and cones. Functional analysis will focus on CTCF, a multifunctional member of the Zn-finger family of transcriptional regulators, which is hypothesized to act as a regulator of retinal cell differentiation through the control of the homeobox gene Pax6. Loss-of-function and gain-of-function experiments in vivo and in vitro will be used for these studies. The transcription factor profiles of differentiated rods and cones will then be investigated, and will be compared with the profiles of their embryonic progenitors as they mature during development. These studies will involve the isolation of embryonic retinal cell layers and individual rods and cones, and their analysis with custom-made transcription factor microarrays and quantitative PCR. Temporal and spatial patterns of expression of candidate genes thus identified will be further investigated by in situ hybridization and immunocytochemisty in retinal sections. The studies are expected to identify common as well as distinctive features in the transcription factor profiles of different photoreceptor subtypes and their progenitor cells, and to provide insights into the lineage relationships of rods and cones.